The present invention relates to a structural assembly of part of an aircraft, particularly a rotary-wing aircraft and especially a helicopter, and to an aircraft equipped with such a structural assembly.
The part of the aircraft taken into consideration in this invention is preferably a part intended to be occupied by people, such as the cockpit or the passenger cabin for example. It is known that the structural assembly of such a part of an aircraft comprises, in particular, part of the structure of the fuselage of the aircraft, and cladding panels which are fixed to the interior of this part of the fuselage structure.
It is also known that present inside such a structural assembly is a phenomenon which is particularly troublesome to the occupants, particularly in the case of a rotary-wing aircraft, and that is, noise.
More particularly, with a rotary-wing aircraft it is known that the acoustic spectra defined in the domain between 20 Hz and 20 kHz originate from the superposition of noises of differing origins, which can be grouped into two different groups according to their spectral characteristics, namely pure sounds or spectral-line noise and broadband noise.
In the known way, pure sounds or spectral-line noise occur particularly, as appropriate:
at the characteristic frequencies of the aircraft driveline;
at the sweep frequencies of the (main and tail) rotor blades and at the harmonics of these frequencies;
at the sweep frequencies of the blades of the compressors of the turbine engine units; and/or
at the sweep frequencies of the blades of the fans that cool the main transmission gearbox and/or electrical equipment and at the harmonics of these frequencies,
whereas broadband noise comprises, in particular, as appropriate:
the noise of the boundary layer which grows along the fuselage;
the noise generated by the rotors;
the air inlet and nozzle flow noise;
the engine noise; and/or
the noise of the cockpit or passenger cabin climate-control or heating circuits.
Although all these noises can pose problems, the acoustic annoyance felt by the passengers and crew is caused essentially by the spectral-line noise originating from the main transmission gearbox [arranged between the engine(s) and the (main and tail) rotors] and/or from the engine(s) situated over the cabin, and from noise of aerodynamic origin coming from the main rotor and the air intakes. As a result, the present invention which intends to limit this acoustic annoyance has the objective mainly of reducing said spectral-line noise.
There are various known solutions for reducing such noise inside a rotary-wing aircraft, particularly a helicopter.
It is known that the technique generally employed for reducing, on an industrial scale, the internal noise of a helicopter consists in reducing the level of vibration or the radiation of sources of noise and/or of the fuselage. The design of the appropriate treatments calls upon the following physical mechanisms:
reducing the vibration response by adding highly damping material to the panels that make up the structure;
reducing the acoustic transmission and/or the acoustic radiation of these same panels by modifying the stiffness (use of localized stiffeners or optimization of the stack of layers in the case of a sandwich structure);
introducing acoustic absorption by bonding sound-deadening foams onto one side of the structural panel or cladding or using sachets of glass wool, rockwool, etc.;
having a double-partition effect between a structural panel and its cladding panel; and
using a Helmholtz resonators effect by adding perforated fabrics to the panels.
The first three solutions do actually make it possible to reduce the overall noise level in a cabin over a large range of frequencies but incur a high penalty in terms of mass. In addition, they entail direct treatment of the structure and have therefore to meet numerous requirements: fire resistance, maintenance, etc. Furthermore, the attenuation of the emergence of the spectral-line noise is not sufficient to cause the acoustic annoyance specific to pure sounds to disappear. As to the fifth solution, it does actually allow noise to be reduced in a narrow frequency band, but only for a frequency chosen at the time of design of the panels. The fourth solution on the other hand is far more attractive because it leads to an increase in the acoustic attenuation by virtue of the double partition effect.
Furthermore, it is known that the cladding panels are generally fixed to the structure of the fuselage of a helicopter using blocks. This solution has the disadvantage of not sufficiently attenuating the noise level, because of the numerous blocks that have to be used for this purpose and because of the proximity of some of the blocks to the main lift and forward travel rotor of the helicopter, which is the main source of noise. Furthermore, such a known solution often leaves gaps and discontinuities between the cladding panels, and this of course leads to reduced acoustic insulation.
To at least partially overcome this noise problem, document U.S. Pat. No. 6,158,690, envisages a structural assembly of part of an aircraft, which comprises:
an external structure which corresponds to part of the structure of the fuselage of the aircraft;
a cladding assembly which comprises a rigid framework formed of arcs which are held together by crossmembers, and cladding panels attached to this framework; and
main connecting means, in this instance insulators, for connecting, through an elastic fixing or semi-rigid connection, the framework of said cladding assembly to the side walls of said external structure.
These insulators comprise in particular elements, particularly rings, made of elastomer, that is to say of an elastic material able to reduce vibration, and are arranged toward the bottom of the walls, so as to be distant from the roof where, in the case of the cabin of a rotary-wing aircraft, the lift and forward travel rotor, which is an important noise source, is situated. These various characteristics of the insulators make it possible to reduce the noise inside the cladding assembly.
Furthermore, said cladding assembly is also held by auxiliary means, with respect to the external structure, namely:
at the top of the walls, by elastic buffers which are arranged in such a way as always to be simultaneously in contact with the cladding assembly and with the external structure, but which are fixed only to the external structure and are simply in contact with the cladding assembly; and
at the bottom, by elastic attachments (equipped with elastomer insulating elements) connecting the cladding assembly to the floor of the external structure.
However, this known structural assembly, which thus makes it possible to bring about a reduction in noise, does exhibit several disadvantages:
the reduction in noise remains limited, particularly:
because of the elastic buffers which transmit vibration from the external structure to the framework of the cladding assembly because they are always in contact with these two elements, which vibration is great in the upper part where these buffers are located because of the proximity to the main rotor in the case of a rotary-wing aircraft; and
because of the insulators which, although distant from the main rotor, are nonetheless situated a certain height off the floor, and which above all act directly on the framework which, as is known, because of its construction (arcs connected together by cross members to form a rigid assembly) encourages the transmission of the various vibrations which are sources of noise;
the various connecting means (insulators) and holding means (buffers and attachments) have a short life, because of the presence of the elastic material (elastomer); and
in the event of a crash or violent impact, the retention of the cladding assembly in the upper part is at the very least uncertain because there is no fixing present in the upper part of the cladding assembly, the effectiveness of the buffers (which are fixed only to the external structure) of course being reduced and insufficient in such a situation, and this may lead to very detrimental effects.
It is an object of the present invention to overcome these disadvantages. The invention relates to a structural assembly of part of an aircraft, particularly of a rotary-wing aircraft and which in particular makes it possible to reduce the noise inside this structural assembly.
To this end, according to the invention, said structural assembly of the type comprising:
an external structure which corresponds to part of the structure of the aircraft and is equipped with a floor;
a cladding assembly comprising a number of cladding panels which are joined together rigidly so as to form a self-supporting envelope which has a shape such that it can be mounted, with an intermediate space, inside said outer structure, said self-supporting envelope being arranged inside said outer structure and mounted on said floor; and
a connecting system for connecting said self-supporting envelope to said outer structure,
is notable in that said connecting system comprises a number of connecting means which are formed in such a way as to fix said self-supporting envelope to said outer structure by rigid and permanent connections and to perform this rigid and permanent fixing exclusively between the floor of the outer structure and ends of cladding panels of said self-supporting envelope.
Thus, according to the invention, the self-supporting envelope is fixed only (rigidly and permanently) to the floor, and this makes it possible to reduce the means of transmitting vibration and thus noise. In addition, as in the case of a rotary-wing aircraft, the floor is the part farthest away from the rotor; the vibrations that occur there are those which are attenuated the most.
Furthermore, as this connection at the floor is rigid according to the invention, it is enough to hold the self-supporting envelope in a stable manner on the external structure, even if this connection is limited to one single fixing region (the end of the envelope facing the floor). Such a connection would not have been sufficient with the known connecting means (insulators) or holding means (attachments) anticipated in the aforementioned document U.S. Pat. No. 6,158,690, because all these known means are elastic which means that at least two fixing regions are then necessary in order to obtain holding in a stable fashion.
The present invention has other advantages and, in particular:
a lengthening of life, because of the elimination of any parts made of elastomer;
a reduction in mass, because of the low number of connecting means; and
a reduction in the transmission of vibration, because the connecting means are fixed directly to the cladding panels of the self-supporting envelope rather than to a framework which, as indicated previously, encourages the transmission of vibration.
In a preferred embodiment, at least one of said connecting means comprises a rigid angle bracket with two legs, which is fixed:
by one of its legs to the floor of the outer structure; and
by the other leg, to a cladding panel of the self-supporting envelope.
In the context of the present invention, an angle bracket is to be understood as meaning a bar (particularly made of metal) made up of two legs or flanges joined together at one of their ends, preferably in the shape of an L.
Furthermore, advantageously, said connecting system additionally comprises a number of auxiliary connecting means which are formed in such a way as to create, in the upper part of the self-supporting envelope which is at the opposite end to the end of the self-supporting envelope which faces the floor of the outer structure, regions of connection with said outer structure which is such that they:
allow a freedom of relative movement, throughout space, between the outer structure and the self-supporting envelope, over predefined distances; and
in the case of a relative movement between the outer structure and the self-supporting envelope which exceeds at least one of said predefined distances and which is in at least one predetermined plane in space, stop said relative movement outright.
In consequence, by virtue of the invention:
in the case of normal operation, for which the relative movement between the external structure and the self-supporting envelope remains of course very limited (and smaller than said predefined distances), the freedom for relative movement is total and there is no contact between the external structure and the self-supporting envelope. For such normal operation, the auxiliary connecting means therefore do not transmit vibrations which are sources of noise and are therefore not troublesome; and
in the event of a crash or of a violent impact leading to a significant relative movement between the external structure and the self-supporting envelope, said auxiliary connecting means stop said relative movement outright, that is to say create a rigid connection. This in particular makes it possible to obtain a structural assembly which is more rigid in such a situation and thus to prevent excessive deformation. It will be noted that, in the structural assembly disclosed by document U.S. Pat. No. 6,158,690, there is no rigid connection in the upper part of the envelope, and this of course if very penalizing in the event of a crash.
Advantageously, said connecting system comprises auxiliary connecting means which create regions of connection between practically vertical walls of the outer structure of the self-supporting envelope respectively, in said upper part of the self-supporting envelope and said regions of connection are able to perform stopping in a horizontal plane which is orthogonal to said vertical walls.
In this case, as a preference, at least one of said auxiliary connecting means comprises:
a stud with a head, which stud is fixed to a cladding panel; and
a plate which is fixed to the outer structure facing said stud and which is provided with an opening collaborating with the head of said stud.
Furthermore, advantageously, said connecting system comprises auxiliary connecting means which create regions of connection between practically horizontal walls of the outer structure of the self-supporting envelope respectively, in said upper part of the self-supporting envelope and said regions of connection are able to perform stopping throughout space. This makes it possible to create regions of connection at the roof of the self-supporting envelope, unlike the teachings of document U.S. Pat. No. 6,158,690, which anticipates no connection at roof level.
As a preference, at least one of said auxiliary connecting means comprises:
two collaborating angle brackets, of which one is fixed to the outer structure and the other is fixed to the self-supporting envelope, the free legs of each of these angle brackets being arranged facing one another and being equipped with holes facing one another; and
a peg which passes through said holes, which has a predefined amount of clearance with respect to one of said free legs in the plane thereof, which also has clearance longitudinally along its axis and which has no clearance with respect to the other free leg.
Furthermore, advantageously, said self-supporting envelope comprises means of assembly of at least two cladding panels and each of said assembly means is formed in such a way:
as to allow partial overlap of the two assembled panels along a joining strip;
as to provide a predefined amount of clearance in all the directions of the plane of said joining strip; and
as to prevent any clearance in the direction orthogonal to said plane of the joining strip.
In a preferred embodiment, at least one of said assembly means comprises:
a clip and bridging piece assembly allowing a degree of freedom in translation over a predefined distance in a first direction of said plane of the joining strip;
a mounting plate with oblong holes which collaborate with screws allowing sliding over a predefined distance in a second direction of said plane of the joining strip which differs from said first direction; and
wedging means ensuring stressed contact along the joining strip in the direction orthogonal to the plane of the joining strip.
Thus, by virtue of the invention, the number of parts (supports, nuts and bolts, etc.) for mounting the cladding panels is reduced, and this makes it possible to obtain a saving in mass, to reduce the cost, and to make the various cladding panels of the self-supporting envelope easier to mount and possibly to replace.
In addition, through the overlapping of the cladding panels in accordance with the invention, the gaps and discontinuities, particularly in the roof, are eliminated and the sound-deadening efficiency is improved.